11

FN6708.0

August 14, 2008

many of the basic skills and techniques referenced in the

following. In addition to this guide, Intersil provides complete

reference designs that include schematics, bills of materials,

and example board layouts.

Selecting the LC Output Filter

The duty cycle of an ideal buck converter is a function of the

input and the output voltage. This relationship is written as

shown in Equation 13:

The output inductor peak-to-peak ripple current is written as

shown in Equation 14:

20% to 40% of the maximum DC output load current. The

MOSFET switching loss, inductor core loss, and the resistive

loss of the inductor winding. The DC copper loss of the

inductor can be estimated using Equation 15:

The copper loss can be significant so attention has to be given

to the DCR selection. Another factor to consider when choosing

the inductor is its saturation characteristics at elevated

temperature. A saturated inductor could cause destruction of

circuit components, as well as nuisance OCP faults.

sum of the voltage drop across the capacitor ESR and of the

voltage change stemming from charge moved in and out of

the capacitor. These two voltages are written as Equations 16

and 17:

and:

If the output of the converter has to support a load with high

pulsating current, several capacitors will need to be paralleled

inductance of the capacitor can cause a brief voltage dip if the

load transient has an extremely high slew rate. Low inductance

capacitors should be considered. A capacitor dissipates heat as

shared by a sufficient quantity of paralleled capacitors so that

Take into account that the rated value of a capacitor can fade

as much as 50% as the DC voltage across it increases.

Selection of the Input Capacitor

The important parameters for the bulk input capacitance are

the voltage rating and the RMS current rating. For reliable

operation, select bulk capacitors with voltage and current

ratings above the maximum input voltage and capable of

supplying the RMS current required by the switching circuit.

Their voltage rating should be at least 1.25 times greater

than the maximum input voltage, while a voltage rating of 1.5

times is a preferred rating. Figure 9 is a graph of the input

RMS ripple current, normalized relative to output load current,

as a function of duty cycle that is adjusted for converter

efficiency. The ripple current calculation is written as

expressed in Equation 18:

Where:

- x is a multiplier (0 to 1) corresponding to the inductor

peak-to-peak ripple amplitude expressed as a

- D is the duty cycle that is adjusted to take into account

the efficiency of the converter

Duty cycle is written as expressed in Equation 19:

In addition to the bulk capacitance, some low ESL ceramic

capacitance is recommended to decouple between the drain

of the high-side MOSFET and the source of the low-side

MOSFET.

Selecting The Bootstrap Capacitor

Adding an external capacitor across the BOOT and PHASE

pins completes the bootstrap circuit. We selected the

bootstrap capacitor breakdown voltage to be at least 10V.

Although the theoretical maximum voltage of the capacitor is

excursions below ground by the PHASE node requires that

D

V

O

V

IN

---------

=

(EQ. 13)

(EQ. 14)

I

PP

–

V

O

1D

–

()

⋅

F

⋅

-------------------------------

=

(EQ. 15)

P

COPPER

I

LOAD

2

DCR

⋅

=

ΔV

ESR

I

PP

–

E

⋅ SR

=

(EQ. 16)

ΔV

C

I

PP

–

8C

⋅

SW

⋅

---------------------------------

=

(EQ. 17)

(EQ. 18)

I

IN_RMS

I

MAX

2

DD

2

–

()

⋅

()

xI

MAX

2

D

12

------

⋅⋅

⎝⎠

⎛⎞

+

I

MAX

-----------------------------------------------------------------------------------------------------

=

(EQ. 19)

D

V

O

V

⋅

--------------------------

=

FIGURE 9. NORMALIZED RMS INPUT CURRENT FOR x = 0.8

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

DUTY CYCLE

x = 0.75

x = 0.50

x = 0.25

x = 0

x = 1

ISL62870